1. Field of the Invention
The present invention relates to a photothermographic material and an image forming method, and more particularly to a novel photothermographic material having an excellent printout property and having improved unprocessed stock storability and to an image forming method using the photothermographic material.
2. Description of the Related Art
In recent years, there is a need in the medical field to reduce the amount of waste processing liquids in consideration of environmental conservation and space-saving. For this reason, there is a need for a technology for a photothermographic material for medical diagnosis and for photographic applications, the technology being capable of efficient exposure with a laser image setter or a laser imager and capable of forming a sharp black image with a high resolution and a high sharpness. Such a photothermographic material can eliminate use of processing chemicals in solutions and can provide users with a thermal development system which is simpler and does not destruct the environment.
Although similar requests are present in the ordinary image forming materials, an image for medical use has to have a particularly high image quality with an excellent sharpness and granularity because a precise image is required. Also there is preferred an image of cold black tone in order to facilitate diagnosis. Currently, various hard copy systems utilizing pigments or dyes, such as an ink jet system and an electrophotographic system, are available as ordinary image forming systems, but no such system yet is satisfactory as an output system for medical images.
Thermal image forming systems utilizing an organic silver salt are known. Such systems are disclosed in, for example, U.S. Pat. Nos. 3,152,904 and 3,457,075, and D. Klosterboer, Thermally Processed Silver Systems, ed. Sturge, V. Walworth and A. Shepp (Imaging Processes and Materials, Neblette 8th edition, 1989), chapter. 9, p. 279. More specifically, a photothermographic material generally has a photosensitive layer in which a photocatalyst (for example silver halide) in a catalytically active amount, a reducing agent, a reducible silver salt (for example organic silver salt) and a toning agent for regulating tone of silver if necessary, are dispersed in a matrix binder. The photothermographic material is heated, after image-wise exposure, to a high temperature (for example 80° C. or higher) whereby a black silver image is formed through a redox reaction between the silver halide or the reducible silver salt (acting as an oxidizing agent) and the reducing agent. The redox reaction is accelerated by a catalytic effect of a silver halide latent image, which is formed by the exposure to light. Therefore, a black silver image is formed in an exposed area (see, for example, U.S. Pat. No. 2,910,377 and JP-B No. 43-4924). As a medical image forming system utilizing a photothermographic material, Fuji Medical Dry Imager FM-DPL is available.
A thermal image forming material utilizing an organic silver salt may be prepared by solvent coating, or by coating and drying a coating solution containing an aqueous dispersion of fine polymer particles as a main binder. The latter method can be applied with a simple manufacturing facility and is advantageous to a mass production since solvent recovery and the like are unnecessary.
A photothermographic material generally has an image forming layer in which a photocatalyst (for example silver halide) in a catalytically active amount, a reducing agent, a reducible silver salt (for example organic silver salt), and the like are dispersed in a matrix binder.
A photothermographic material has a big problem that a reducing reaction of silver ions proceeds when exposed to indoor light after an image formation or when exposed to a high temperature during storage, thereby generating a fog. The reducing reaction proceeds because unreacted compounds are unremoved from the photosensitive material after a thermal development process and such compounds remain in the photothermographic material. Consequently, it has been requested to develop a technology for improving image storability.
Regarding a method for improving the image storability, a polyhalogen compound is known to be effective, which oxidatively decompose unnecessary fogging silver which is generated in a processed photothermographic material with time (for example cf. JP-A No. 2001-33911). Also there is disclosed a complex forming agent capable of forming a complex with a developing agent, thereby suppressing an undesirable reducing reaction during storage (for example cf. JP-A Nos. 2002-156727 and 2002-318431). However, even these technologies can reduce a “printout”, an increase in the fog level under an exposure to light, only insufficiently, and a further improvement is therefore requested.
As a result of intensive investigation, it was found that the printout could be significantly improved by using a silver iodide emulsion as a photosensitive silver halide (for example cf. JP-A No. 2001-209145 and EP Nos. 1276006 and 1276007). However, it has been found that a silver iodide emulsion involves a problem of a sensitivity loss in a photothermographic material during a storage period from a manufacture thereof to a laser exposure or a thermal development for image formation. Since such sensitivity loss becomes larger as a temperature or a humidity during storage becomes higher, and the sensitivity loss increases with time, it is not possible to obtain a constant sensitivity in use. Further, it is unable to predict the tendency of sensitivity loss in advance, thus there is a need for a stabilization of the sensitivity.